Conventionally, there has been known a resin-made tank which includes: a resin-made tank body; a resin-made cylindrical filling portion for liquid filling, the cylindrical filling portion provided on the tank body; a mouthpiece engageable with the cylindrical filling portion; and a seal member disposed between the mouthpiece and the cylindrical filling portion (see Patent Literature 1, for example). In Patent Literature 1, a clip portion positioned below the seal member is mounted on a lower end portion of the mouthpiece, and the mouthpiece is fixed by swaging the clip portion to an outer peripheral portion of the cylindrical filling portion.